1. Field of the Invention
This invention relates to a method and apparatus for obtaining radiographs, and more particularly to such a method and apparatus by which X-ray images can be obtained by imagewise exposing a cloud chamber containing a high atomic number gas mixed with a condensate vapor.
2. Description of the Prior Art
Cloud chambers, first devised by C. T. R. Wilson, have long been used to render visible the paths of ionizing particles. Such early chambers contained saturated water vapor and air. By allowing the mixture to expand quickly (adiabatically), the vapor cools and water droplets condense on any particles which serve a nuclei of condensation. An ionizing particle moving rapidly in the water vapor creates many ions and electrons in its path. Water droplets condense on these ions and electrons and thus make visible the path of the ionizing particle. This effect is totally obscured at slightly greater expansions by the copious condensation upon uncharged aggregates of molecules.
Such cloud chambers exhibited a serious disadvantage; a long recovery time. In 1949, it was found that by following the expansion and drop-growth interval with an overcompression, the gas was heated and the re-evaporation of the drops speeded.
U.S. Pat. No. 2,899,557, which issued Aug. 11, 1959 to Robert R. Wilson, discloses an apparatus for obtaining shadowgraphs or radiographs of an object exposed to X-rays by use of a cloud chamber, and for photographically recording the radiograph thus produced. An object to be radiographed is placed between an X-ray source and a cloud chamber having a pair of spaced end walls. The inner surface of the end wall closest to the X-ray source is coated with lead glass. X-rays which pass through the object impinge on the lead glass plate and cause secondary electron emission from the lead glass, varying in intensity inversely with the density of the object under observation, through the cloud chamber to produce vapor trails as explained hereinbefore. Since the electron emission is dependent upon the number of X-rays reaching the lead glass plate, and the intensity of X-rays vary throughout the image in proportion to the character of the object being radiographed, the cloud chamber image is therefore indicative of the physical character of the object. The image produced within the chamber may be observed or photographed through the wall opposite the X-ray source.
Although the apparatus just described produces acceptable radiographs, the electrons emerging from the lead glass plate have excessive range in the relatively low density gas between the plates and therefore cannot produce sharp images. Further, the apparatus' quantum efficiency (the fraction of X-ray photons absorbed by a device which lead to a detectable event) is quite low, requiring excessive X-ray dosages. The lead glass (or other heavy metal or metal coating), upon absorbing X-rays, emits both primary photoelectrons from the tightly bound K, L or M shells as well as secondary electrons caused by the ionizing effects of the primary electrons within the metal. The primary electrons have high energy (tens of thousands of electron volts), and they create many secondary electrons within the metal, thereby losing their energy. Most of the primary electrons lose so much energy in the process of secondary ionization that they are trapped inside the metal coating. A very few primaries lose only a small amount of energy and are emitted into the cloud chamber. The second electrons created in the metal coating by the primaries are also mostly unable to exit from the coating surface.
Other prior art radiographic devices employ film in conjunction with intensifying screens. But even these devices exhibit quantum efficiencies of only around 20 to 40 percent. Further, the screens are rather expensive, contact with the film is not too good and the screens are easily damaged during use.